Electronic Materials
• The goal of electronic materials is to generate and control the flow of an electric current.
• Electronic materials include:1. Conductors: have low resistance which
allows electric current flow2. Insulators: have high resistance which
suppresses electric current flow3. Semiconductors: can allow or suppress
electrical current flow
Insulators
Insulators have tightly bound electrons in their outer shellThese electrons require a very large amount of energy to free them for conduction
Let’s apply a potential difference across the insulator above…
The force on each electron is not enough to free it from its orbit and the insulator does not conduct
Insulators are said to have a high resistivity / resistance
Insulators
• Insulators have a high resistance so current does not flow in them.
• Good insulators include:– Glass, ceramic, plastics, & wood
• Most insulators are compounds of several elements.
• The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.
Conductors
Conductors have loosely bound electrons in their outer shellThese electrons require a small amount of energy to free them for conduction
Let’s apply a potential difference across the conductor above…
The force on each electron is enough to free it from its orbit and it can jump from atom to atom – the conductor conducts
Conductors are said to have a low resistivity / resistance
Conductors
• Good conductors have low resistance so electrons flow through them with ease.
• Best element conductors include:– Copper, silver, gold, aluminum, & nickel
• Alloys are also good conductors:– Brass & steel
• Good conductors can also be liquid:– Salt water
Conductor Atomic Structure• The atomic structure of
good conductors usually includes only one electron in their outer shell. – It is called a valence electron. – It is easily striped from the
atom, producing current flow.
Copper Atom
Semiconductors• A material whose properties are such that it is not quite a
conductor, not quite an insulator.• Semiconductors have a resistivity/resistance between that of
conductors and insulators.• Their electrons are not free to move but a little energy will free them
for conduction• Some common semiconductors
– elemental• Si - Silicon (most common)• Ge - Germanium
– compound• GaAs - Gallium arsenide• GaP - Gallium phosphide• AlAs - Aluminum arsenide• AlP - Aluminum phosphide• InP - Indium Phosphide
(The resistance of a semiconductor decreases as the temperature increases.)
The Silicon, Si, AtomSilicon has a valency of 4 i.e. 4 electrons in its outer shell
Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms
These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms
This picture shows the shared electrons
Silicon – the crystal latticeIf we extend this arrangement throughout a piece of silicon…
We have the crystal lattice of silicon
This is how silicon looks when it is cold
It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator
Electron Movement in SiliconHowever, if we apply a little heat to the silicon….
An electron may gain enough energy to break free of its bond…
It is then available for conduction and is free to travel throughout the material
Hole Movement in SiliconLet’s take a closer look at what the electron has left behind
There is a gap in the bond – what we call a hole
Hole Movement in SiliconThis hole can also move…
An electron – in a nearby bond – may jump into this hole…
Effectively causing the hole to move…
Like this…
Semiconductor Valence Orbit
• The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.
Crystal Lattice Structure• The unique capability of
semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice.
• The atoms link together with one another sharing their outer electrons.
• These links are called covalent bonds. 2D Crystal Lattice
Structure
Semiconductors
• An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present.
• An extrinsic semiconductor is a semiconductor that has been doped.
Doping• Relying on heat or light for conduction does not
make for reliable electronics• To make the semiconductor conduct electricity,
other atoms called impurities must be added.• “Impurities” are different elements. • This process is called doping.
Semiconductors can be Conductors
• An impurity, or element like arsenic, has 5 valence electrons.
• Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms.
• The one electron left over for each arsenic atom becomes available to conduct current flow.
The Phosphorus AtomPhosphorus is number 15 in the periodic table
It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell
Doping – Making n-type SiliconSuppose we remove a silicon atom from the crystal lattice…
and replace it with a phosphorus atom
We now have an electron that is not bonded – it is thus free for conduction
Doping – Making n-type SiliconLet’s remove another silicon atom…
and replace it with a phosphorus atom
As more electrons are available for conduction we have increased the conductivity of the material
If we now apply a potential difference across the silicon…
Phosphorus is called the dopant
Extrinsic Conduction – n-type Silicon
A current will flow
Note:
The negative electrons move towards the positive terminal
From now on n-type will be shown like this.
N-type Silicon
This type of silicon is called n-type This is because the majority charge carriers are
negative electrons A small number of minority charge carriers – holes –
will exist due to electrons-hole pairs being created in the silicon atoms due to heat
The silicon is still electrically neutral as the number of protons is equal to the number of electrons
The Boron AtomBoron is number 5 in the periodic table
It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell
Doping – Making p-type SiliconAs before, we remove a silicon atom from the crystal lattice…
This time we replace it with a boron atom
Notice we have a hole in a bond – this hole is thus free for conduction
Doping – Making p-type SiliconLet’s remove another silicon atom…
and replace it with another boron atom
As more holes are available for conduction we have increased the conductivity of the material
If we now apply a potential difference across the silicon…
Boron is the dopant in this case
P-type Silicon
This type of silicon is called p-type This is because the majority charge carriers are positive
holes A small number of minority charge carriers – electrons –
will exist due to electrons-hole pairs being created in the silicon atoms due to heat
The silicon is still electrically neutral as the number of protons is equal to the number of electrons
From now on p-type will be shown like this.
The p-n Junction
Suppose we join a piece of p-type silicon to a piece of n-type silicon
We get what is called a p-n junction
Remember – both pieces are electrically neutral
The p-n JunctionWhen initially joined electrons from the n-type migrate into the p-type – less electron density there
When an electron fills a hole – both the electron and hole disappear as the gap in the bond is filled
This leaves a region with no free charge carriers – the depletion layer – this layer acts as an insulator
The p-n JunctionAs the p-type has gained electrons – it is left with an overall negative charge…
As the n-type has lost electrons – it is left with an overall positive charge…
Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V
0.6 V
The Reverse Biased P-N JunctionTake a p-n junction
Apply a voltage across it with the
p-type negative
n-type positive
Close the switch
The voltage sets up an electric field throughout the junction
The junction is said to be reverse – biased
The Reverse Biased P-N JunctionNegative electrons in the n-type feel an attractive force which pulls them away from the depletion layer
Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer
Thus, the depletion layer ( INSULATOR ) is widened and no current flows through thep-n junction
The Forward Biased P-N JunctionTake a p-n junction
Apply a voltage across it with the
p-type postitive
n-type negative
Close the switch
The voltage sets up an electric field throughout the junction
The junction is said to be forward – biased
The Forward Biased P-N JunctionNegative electrons in the n-type feel a repulsive force which pushes them into the depletion layer
Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer
Therefore, the depletion layer is eliminated and a current flows through the p-n junction
The Forward Biased P-N JunctionAt the junction electrons fill holes
They are replenished by the external cell and current flows
Both disappear as they are no longer free for conduction
This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V
The Forward Biased P-N JunctionIf we apply a higher voltage…
The electrons feel a greater force and move faster
The current will be greater and will look like
The p-n junction is called a DIODE and is represented by the symbol…
The arrow shows the direction in which it conducts current
this….